The demands and requirements of data communication networks are becoming increasingly complex. For example, it is increasingly desired for data networks to be capable of providing efficient support for communication sessions with different Quality of Service parameters, for network mobility etc. There is furthermore a desire that this functionality is supported by popular data communication protocols such as the Internet Protocol (IP).
Indeed there is a desire for conventional data protocols such as IP to be used for an increasing number of different applications and systems.
For example, in the future evolution of cellular communication systems, it is expected that these will increasingly be based on IP traffic. For example, it is envisaged that a substantial part of the voice communication will be supported by Voice over IP (VoIP) traffic in the future. Accordingly, the 3rd Generation Partnership Project (3GPP), which is responsible for standardizing the 3rd Generation cellular communication systems, has been standardizing a network architecture that supports IP traffic. This architecture is compatible with and supplements the traditional cellular network architecture and is known as the IP Multimedia Subsystem (IMS).
The aim of IMS is not only to provide new services but also to provide all the services that the Internet provides. In addition, it is desired that users are able to execute all their services when roaming as well as from their home networks. To achieve these goals, IMS uses open standard IP protocols, defined by the Internet Engineering Task Force (IETF). As a consequence multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet can be established using exactly the same protocol(s).
In particular, IMS uses a VoIP implementation based on a 3GPP standardized implementation of the Session Initiation Protocol (SIP) and runs over the standard Internet Protocol (IP). Existing phone systems (both packet-switched and circuit-switched) are supported.
SIP is a standard for initiating, modifying, and terminating an interactive user session that involves multimedia elements such as video, voice, instant messaging, online games, and virtual reality. SIP is only used in setting up and tearing down e.g. voice or video calls whereas the voice or video communications themselves are carried out using the Real-time Transport Protocol (RTP).
A goal for SIP is to provide a superset of the call processing functions and features present in the Public Switched Telephone Network (PSTN). As such, features that permit familiar telephone-like operations are present: e.g. dialing a number, causing a phone to ring, hearing ringback tones or a busy signal etc.
SIP also implements many more advanced call processing features. Furthermore, SIP is a peer-to-peer protocol and therefore requires only a very simple (and thus highly scalable) core network with intelligence distributed to the network edge and embedded in the endpoints (the originating or terminating devices). Many SIP features are implemented in the communicating endpoints.
IMS supports functionality for managing and controlling subscription information for the users of the system. Specifically, an IMS network comprises a Home Subscriber Server (HSS) which is a master user database that supports the IMS network entities that are actually handling the calls/sessions. These entities comprise the so-called Call Server Control Function (CSCF) elements. Specifically, a Serving-CSCF (S-CSCF) is a central node of the signaling plane and acts as a SIP server and performs session control for the user equipments served by it. Furthermore, mobility is supported by Proxy-CSCFs (P-CSCFs) which are SIP proxies that provide the first point of contact for a mobile user equipment. P-CSCFs may be located either in the visited network (e.g. in full IMS networks) or in the home network (e.g. when the visited network is not IMS compliant).
In IMS and similar systems, the SIP protocol is used to initialize call sessions between an originating node and a terminating node. This call session setup involves the originating node initializing a SIP INVITE message which is forwarded to CSCFs supporting the originating node and from these to a serving CSCF supporting the terminating node (the serving CSCP may be the same for the originating and terminating user equipment). This serving CSCF forwards the SIP INVITE message to a proxy-CSCF supporting the terminating node and from this it is forwarded to the terminating node. The terminating node may then accept the SIP INVITE and the SIP call session setup process may continue. The proxy-CSCF is furthermore coupled to a policy manager which is arranged to reserve resources for the call setup process.
However, although the approach of the SIP setup process may be advantageous in many scenarios, there are also some associated disadvantages. Specifically, in some scenarios the call session setup approach may result in a suboptimal resource usage wherein e.g. the call session setup will reserve unnecessary and unused resource for the call session.
In particular, the SIP protocol allows forking of the call sessions. A forking of a call session comprises the call session from the originating node being forked to a plurality of different potential terminating nodes. Each of these terminating nodes may accept the call session setup but the resulting call session will only be with one terminating node. Thus, the call session being setup will be between one originating node and one terminating node although the specific terminating node may be one of the plurality of possible terminating nodes. However, in current approaches, the SIP call session setup for forked call sessions will result in resource being reserved for all possible terminating nodes thereby resulting in an excessive resource usage and thus a reduced capacity of the data network as a whole.
Hence, an improved data network would be advantageous and in particular a data network that allows improved call session setup, reduced call session resource usage, facilitated operation, facilitated implementation and/or improved performance would be advantageous.